Chapter 14 - Mass Spectrometry

Mass spectrometry is an analytical technique for determining the mass-to-charge ratio (m/z) of ions, which were previously positive but are now both positive and negative. * %%Mass spectrometry concepts were first identified in 1898.%%
Thomson found that neon can be split into a more abundant isotope, 20Ne, and a less abundant isotope, 22Ne, when he recorded the first mass spectrum in 1911. F. W. * %%Aston demonstrated, using improved instruments, that the majority of naturally occurring elements are isotope mixes.%%
%%It was discovered, for example, that around 75% of chlorine atoms in nature are 35Cl and 25% are 37Cl.%% * Mass spectrometry did not become widely used until the 1950s, when commercial equipment with good resolution, dependability, and low maintenance costs were available. * ^^Mass spectrometry is now our most important analytical instrument for determining precise molecular masses.^^ * Furthermore, study of a compound's mass spectrum can provide detailed information about its molecular formula and structure. * Mass spectrometry is also becoming increasingly essential in biochemistry; protein structures may be determined on a nearly single-cell size using this approach alone. * Mass spectrum: refers to a plot of the relative abundance of ions verus their mass to charge (m/z) ratio.
There are several varieties of mass spectrometers; we have room in this article to explain only the most basic. * A vaporized sample in an evacuated ionization chamber is attacked with high-energy electrons, causing electrons to be stripped off the sample's molecules, yielding positively charged ions in first-generation spectrometers. * Radical anions (molecules with an additional electron) are increasingly being explored; nonetheless, these are beyond the scope of this work. * Positive ions are accelerated into an analyzing chamber by a sequence of negatively charged accelerator plates inside a magnetic (electric in certain spectrometers) field perpendicular to the direction of the ion stream. * ^^The ion beam curves due to the magnetic field. Each ion's radius of curvature is determined by^^
Gas and volatile liquid samples can be put directly into the ionization chamber. * %%Volatile liquids and even certain solids are vaporized because the inside of a mass spectrometer is maintained at a high vacuum.%%
For less volatile liquids and solids, place the sample on the tip of a heated probe, which is then inserted straight into the ionization chamber. * Connecting a gas chromatograph (GC) or liquid chromatograph (LC) directly to the mass spectrometer is another incredibly effective approach for bringing a sample into the ionization chamber. * These devices are capable of separating complicated molecular mixtures into pure fractions. * Each fraction eluted from the chromatograph is directly injected into the mass spectrometer's ionization chamber, allowing mass measurement of the constituent components.
A molecular ion, M1, is the species created when a single electron is removed from a molecule. * A molecular ion is a type of ion known as a radical cation. * %%When methane is blasted with high-energy electrons, for example, an electron is dislodged from the molecule, yielding a molecular ion with m/z 16.%%

The ionization potential of the atom or molecule determines which electron is lost in the formation of the molecular ion.
Most organic compounds have ionization potentials between 8 and 15 eV. * ^^The potentials are at the lower end of this range for both nonbonding oxygen and nitrogen electrons and p electrons in unsaturated compounds such as alkenes, alkynes, and aromatic hydrocarbons.^^ * Ionization potentials for s electrons, such as those found in C!C, C!H, and C!O s bonds, are at the upper end of the spectrum.
It makes no difference which electron is lost for our purposes because the radical cation is delocalized throughout the molecule. * %%As a result, we put the parent molecule's molecular formula in brackets with a plus sign.%%
As shown in the image attached, a molecular ion can be fragmented to generate a variety of smaller cations (which can then be fragmented further), radicals, and smaller molecules. * Only charged bits are picked up.
Following the formation of molecular ions and their fragments, a positively charged repeller plate guides the ions toward a sequence of negatively charged accelerator plates, resulting in a quickly moving ion beam.
The ion beam is then concentrated by one or more slits before entering a mass analyzer and entering a magnetic field perpendicular to the ion beam's direction. * The ion beam curves due to the magnetic field.
Cations with higher m/z values are deflected less than those with lower m/z values. By altering either * Base peak: refers to the peak caused by the most abundant ion in a mass spectrum; the most intense peak. It is assigned an arbitrary intensity of 100
The method we described is known as electron ionization mass spectrometry (EI-MS). * This was the first approach devised and, for a time, the most extensively employed. * However, it is restricted to relatively low-molecular-weight molecules that evaporate quickly in the evacuated ionization chamber.
%%A breakthrough in ionization methods in recent years has expanded the application of mass spectrometry to very high molecular-weight substances and others that cannot be directly evaporated.%% * Fast-atom bombardment (FAB) is one of the novel techniques that employs high-energy particles, such as xenon atoms accelerated to keV energies, to bombard a dispersion of a chemical in a nonvolatile matrix, creating ions of the compound and expelling them into the gas phase. * A second method is matrix-assisted laser desorption ionization mass spectrometry.